Heretofore, various attempts to improve the rebound performance of a golf club head have been made in order to increase the flight distance of the ball.
The following Patent Documents 1 and 2 disclose hollow golf club heads, wherein a wall defining the top portion (or crown portion) of the head is provided with a flexure protruding toward the inner cavity of the head.    Patent Document 1: Japanese Patent Application Publication No. 2002-52099    Patent Document 2: Japanese Patent Application Publication No. 2014-180540